CHAPTER 6

Lesson 1 (a)

1/12 Definition and Mathematics of Work

• When a force acts upon an object to cause a displacement of the object, it i s said that work was done upon the object.
• Three things for work:
  • force
  • displacement : in order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement.
  • cause
• W = F*d*(costheta)
• The angle measure of this is defined as the angle between the force and the displacement.
• On occasion, a force acts upon a moving object to hinder a displacement. In such instances, the force acts in the direction opposite the objects motion in order to slow it down.
  • The force doesn't cause the displacement.
• These situations involve what is commonly called negative work. The negative of negative work refers to the numerical value that results when values of F, d and theta are substituted into the work equation. Since the force vector is directly opposite the displacement vector, theta is 180 degrees. The cosine(180 degrees) is -1 and so a negative value results for the amount of work done upon the object.

Calculating the amount of work done by forces:

• When a force acts to cause an object to be displaced, three quantities must be known in order to calculate the work.
  • force
  • displacement
  • the angle between the force and the displacement
• work = (force)(displacement)(costheta)
  • theta is the angle between the force and the displacement vectors.

Lesson 2 (a)

1/30 Class Notes:


Identifying Energy Types Exercise:
1) PE to KE

• correct

2) PE to KE

• correct

3) KE to PE

• correct

4) KE to PE

• correct

5) PE to KE

• correct

6) Positive work (correct) PE (wrong)
7) Positive work (correct) Both (correct)
8) Positive work (correct) KE (correct)
9) Negative work (correct) KE (correct)
10) Positive work (correct) PE (wrong)
11) Positive work (correct) Both (wrong)


Internal vs. External Forces - Physics Classroom
Certain forces when present and involved in doing work on objects will change the total mechanical energy of the object. there are other types of forces that can transform the energy of an object from potential energy to kinetic energy (or vice versa)called internal forces and external forces.
External Force:

• includes the applied force, normal force, friction force, and air resistance
• Fapp, Ffrict, Fair, Ftens, Fnorm

Internal Froce:

• include the gravity forces, magnetic force, electrical force, and spring force
• Fgrav, Fspring

Characterizing them:

• based on whether or not their presence is capable of changing an object's total mechanical energy.
• when net work is done upon an object by an external force, the total mechanical energy of that object is changed
  • positive work --> object will gain energy
  • negative work --> object will lose energy
  • - When the only type of force doing net work upon an object is an internal force, the total mechanical energy of that object remains constant
    - In such cases, the object's energy changes form
    - This is referred to as energy conservation
    - When the only forces doing work are internal forces, energy changes forms from kinetic to potential or vice versa

Energy Bill Analysis

2/9/12
Objective
Estimate how much electrical energy you consume on a daily basis (ie: what does it cost my parents)
What to do?

1. Identify 10 electrical appliances you use on a daily basis and identify their power rating (in Watts)

• TV = 70 W = .07 kW
• Hair Straightener = 50 W = .05 kW
• Desk Lamp = 40 W = .04 kW
• Blackberry Charger = 15 W = .015 kW
• Hair Dryer = 1875 W = 1.875 kW
• Pencil Sharpener = 20 W = .02 kW
• Apple Laptop Charger = 30 W = .03 kW
• Microwave = 950 W = .95 kW
• DVD Player = 12 W = .012 kW
• Desktop Computer = 120 W = .12kW

Equation Used =

1. The power rating will be listed on the info label – typically on the back of or under the appliance. Often the power consumption is listed in terms of AMPERAGE (amperage measures the current of electricity consumed). The power rating (in Watts) may be calculated using the following formula:

Power (in Watts) = AMPS x 120 volts.

For example, a 12 AMP vacuum cleaner uses 12A x 120v = 1440 Watts of power.

1. Determine the total kWh (kilowatt-hours) of power that you consume with these ten appliances.

1. First estimate how many hours you use each appliance. For example, you may use your hairdryer every second day for 10 minutes – so on average you use it for 0.17h/d ¸ 2 = 0.085 h per day.

• TV = 2h/d
• HS = 0.17h/d
• Desk Lamp = 2h/d
• BBC = 2h/d
• HD = 0.17h/d
• PS = 2m/d
• Comp Charger = 10h/d
• Microwave = .25h/d
• DVD Player = 2h/d
• Desktop Comp = 2h/d

1. Calculate the kWh used per day by multiplying the wattage of the appliance by the hours used (per day).

Equation:

1. TV= .14 kWh
2. Hair Straightener = .0085 kWh
3. Desk Lamp = .08 kWh
4. Blackberry Charger = .013 kWh
5. Hair Dryer = .31875 kWh
6. Pencil Sharpener= .04 kWh
7. Apple Laptop Charger = .3 kWh
8. Microwave = .2375 kWh
9. DVD Player = .024 kWh
10. Desktop Computer = .24 kWh

1. Finally, determine the total kWh you use per day.
• 1.40175 kWh

1. What does this cost your parents?

• 1418.75 * ($0.119175) = $169.08

1. Look at your household electric bill. Find two numbers:

1. the total dollar amount due
• $275.55
1. kWh consumed that month.
• 1479 kWh

1. Costs of Electricity.
   1. Calculate the average cost per kWh by dividing the dollar amount by the kWh consumed. This is approximately what each kWh costs your family. (Note: this figure includes fees and taxes, too). The cost will probably be between $0.10-$0.20 per kWh – depending on how much electricity you use. Electricity costs more if you exceed a certain usage.
      • $0.186308
   2. You can also look this value up on the bill itself.
      • $0.119175
   3. Compare the two values. Why are they different?
      • They are probably different because the company, PSE&G is trying to make money off of each customer.

1. Multiply the kWh you use per day by the cost per kWh to determine what you cost your parents.
   • 1.40175 kWh * $0.119175 = $ 0.16705

Evaluate

1. Does the cost of electricity seem like a lot to you? Discuss this with your friends.
   • The cost of electricity today does not seem like a lot compared to other things, but after time it probably adds up.

1. Keep in mind that this cost only reflects ten appliances. What do you think your total cost actually is? Are there major appliances that you may have excluded from your list of ten that would make a big difference?
   • I'm sure that my total cost is a little bit higher, but not by that much. I definitely use a large amount of energy There are many appliances that I left out from my list. I forgot to include the refrigerator, garage door, dish washer, dryer, and the gas fireplace.

1. If you think your consumption is a lot – what can you change? Do you need/should change?
   • After looking at the amount of kWh my family uses, I only use a small portion of this total amount. However, I can still do some things differently to decrease this number. I should stop leaving my phone and computer charger plugged in all day, while I am at school. The one thing I have improved on the past couple of months is remembering to turn the lights off when I leave the room. Also, I now shut my TV off before going to bed at night, instead of leaving it on all night, like I used to.

According to my calculations from this activity, I really am not costing my dad that much. After looking at the bill, and calculating everything I realized that it all adds up. For a family of four, it costs us around $275 per month on electricity. I cannot imagine what the bill would cost for a large family, or families living in huge houses. My mom has been telling me for a while to shut the lights off when I leave the room, and over the past couple of months I have really started to listen and realize how important it is.